The present invention relates to an improved trimmer head for use in flexible line rotary trimming devices used to trim grass, weeds and other vegetation. More particularly, the invention is directed to an improvement in the aerodynamics of the trimmer head so that with the same drive input, the head will rotate faster than a conventional trimmer head, enhancing the cutting efficiency of the rapidly rotating cutting line or rotate at the same speed as a conventional head with less drive input, saving energy. The improved aerodynamics of the head in the present invention also has been found to reduce vibration and noise during use.
Trimmer heads used in flexible line rotary trimmers generally carry one or two lengths of flexible nylon cutting line typically wrapped about an interior spool with the ends of the line or lines projecting outwardly through opposed apertures in the side wall of the trimmer head. The head is threadably mounted on the end of an elongated shaft and rotated at a high velocity by a gas or electric motor so that the ends of the cutting line project radially from the head and sever weeds or other vegetation. When cutting line projecting from the head breaks off or becomes overly worn, it must be severed and fresh line extended from the spool through the line outlet eyelets in the side of the housing. Bump-feed type heads such as those disclosed in U.S. Pat. Nos. 4,458,419, 4,959,904 and 6,901,667, include a line feed-out mechanism which responds to the operator intentionally bumping the rotating head against the ground during use to feed out a measured length of fresh cutting line which is typically cut to the desired length by a knife blade projecting from a shield attached to the trimmer above the cutting head and spaced a predetermined distance from the perimeter of the trimmer head housing. Manual heads such as that disclosed in U.S. Pat. No. 7,275,324, do not include any such line feed-out mechanism. The spool must be manually rotated relative to the housing to pay out additional cutting line. Automatic heads such as that disclosed in U.S. Pat. No. 5,063,673, include a mechanism that detects a loss of mass in the cutting line projecting from the head during use and, without the need for any action by the operator, pay out additional lengths of line from the spool through the eyelets. Fixed line heads such as that disclosed in U.S. Pat. No. 6,928,741, utilize one or more relatively short lengths of cutting line, typically under 12 inches in length, that project from the trimmer head housing and are held proximate their inner ends by a line gripping mechanism disposed within the housing. The improvement of the present invention is adaptable for use with each of these types of head to improve the efficiency thereof and reduce noise and vibration.